Field of the Invention
The present invention relates to an image processing apparatus, image processing method, and storage medium for performing a quantization process to form an image on a print medium.
Description of the Related Art
In the case of using a pseudo gradation method to print an image, it is necessary to quantize multi-valued image data, and as a quantization method used for the quantization, an error diffusion method and a dither method are known. In particular, the dither method that compares a preliminarily stored threshold value and a gradation value of multi-valued data with each other to determine dot printing or non-printing has a small processing load as compared with the error diffusion method, and is therefore widely used in many image processing apparatuses. In the case of such a dither method, in particular, dot dispersibility in a low gradation range becomes problematic; however, as a threshold value matrix for obtaining preferable dot dispersibility, a threshold value matrix having blue noise characteristics is proposed.
FIGS. 14A to 14C are diagrams for explaining a dither process using a threshold value matrix having blue noise characteristics. FIG. 14A illustrates an example of image data to be inputted into a 10-pixel×10-pixel area. This example shows a state where a gradation value of “36” is inputted to all the pixels. FIG. 14B illustrates a threshold value matrix prepared corresponding to the above 10-pixel×10-pixel area. Each of the pixels is related to any of threshold values of 0 to 254. In the dither method, in the case where a gradation value indicated by multi-valued image data is larger than a threshold value, a corresponding pixel is designated as dot printing “1”. On the other hand, in the case where a gradation value indicated by multi-valued image data is equal to or less than a threshold value, a corresponding pixel is designated as dot non-printing “0”. FIG. 14C illustrates a quantization result based on the dither method. Pixels representing printing “1” are indicated in gray, and pixels representing non-printing “0” are indicated in white. The distribution of printing “1” pixels as seen in FIG. 14C is changed depending on threshold value arrangement in a threshold value matrix. By using the threshold value matrix having blue noise characteristics as in FIG. 14B, even in the case where the same pieces of multi-valued data are inputted into a predetermined area as in FIG. 14A, the printing “1” pixels are arranged in a high dispersibility state as in FIG. 14C. In addition, when a person visually observes such an image, dot deviation or periodicity is unlikely to be perceived, and the image can be recognized as a comfortable image.
However, even in the case of using a threshold value matrix having blue noise characteristics, in the case of printing an image using multiple color materials (i.e., mixed color), dispersibility is lost to make graininess conspicuous in some cases.
U.S. Pat. No. 6,867,884 discloses a dither method for solving such a problem. Specifically, U.S. Pat. No. 6,867,884 discloses a method that prepares one common dither matrix having preferable dispersibility, and performs a quantization process while shifting mutual threshold values among multiple colors. According to U.S. Pat. No. 6,867,884 as described, dots of different colors are mutually exclusively printed in a high dispersibility state in a low gradation range, and therefore even in the case of a mixed color image, preferable image quality can be achieved.
However, the method disclosed in U.S. Pat. No. 6,867,884 focuses on graininess and dispersibility in a mixed color image, but does not focus on any defect associated with print position displacement among color materials. That is, in a configuration where a print head is prepared for each of color materials to be printed, if relative print position displacement occurs among the print heads due to some cause, density unevenness and color unevenness are caused on an image. In addition, such unevenness is repeated with a period corresponding to the size of a dither matrix, and easily recognized as image defects.